J. 0. Thompson — Law of Elastic Lengthening. 39 



Preceding 











Tension. 



Change 



from 2-13 sec. 



From 13-30. 



Total change. 



kg. 





mm. 



mm. 



mm. 



0-6 





0-035 



o-ooi 



0-036 



1-2 





0-062 



o-oio 



0-072 



1-8 





o-ioo 



0-020 



0-120 



The figures in each column are mean values of 10, 20 and 

 8 measurements respectively. That the changes given in the 

 second column come mostly from the thermal effects within 

 the wire itself and not from the after-effect can be proved in 

 the following way : If we insert in the equation which we 

 have already mentioned in place of A the value ^-g^y as ob- 

 tained by Edlund we get 



1 To'- . 



At— A p. 



682-7 w c * 



In these experiments T = 285°, a=0000018, w=(H)00528kg. 

 per m., and c= 0*094. Consequently according to the equation 

 j£=0 o, 15 when Jj)=l kg. Since the wire was 22 - 7 m. long 

 the change in its length for 1° was 0*409 mm., or 0'0614 mm. 

 when At was 0°'15. Therefore on removing stretching weights 

 of 0'6, l - 2 and 1*8 kg. the lengthenings of the wire caused by 

 the evolution of heat in the first instant should be 0'037, - 074 

 and O'lll mm. respectively. These figures approximate those 

 in the second column of the above table as closely as we could 

 expect in an experiment of this kind. 



From what has been said it is safe to conclude that the 

 necessary correction on account of the after-effect is very small. 

 The stretching of the wire caused by a weight of 1*2 kg. was 

 43*55 mm., and the change of 001 mm. which took place after 

 the first 13 sec. is insignificant in comparison. 



Method of Measurement. 



The method followed in making all measurements was the 

 same. After noting the position of the mark on the thermom- 

 eter-wire the telescopes of both cathetometers were sighted on 

 the mark on the main-wire, and careful readings were made. 

 Then a weight of 0*2 kg. was put on the pan, the consequent 

 lowering of the mark was quickly measured by cathetometer I, 

 and then the weight was immediately removed. 



Two minutes later any shifting of the zero-point was meas- 

 ured by means of the micrometer screw of cathetometer II, 

 and then the same process was repeated with a weight of 04 

 kg. In some cases the weight was increased to as much as 

 1-8 kg. 



